The present invention relates generally to hydrostatic transmission control systems, and more particularly, to an electrical steering control system for a dual path hydrostatic drive vehicle.
The use of hydrostatic transmissions to drive off-highway vehicles has become increasingly popular for many reasons, such as the ability to infinitely vary the speed ratio. A somewhat unique application is the use of a pair of hydrostatic transmissions to drive a dual path (or dual track) vehicle, such as a crawler tractor.
In dual path hydrostatic drive systems, it is necessary to precisely match the speeds of the two transmissions to accomplish "straight tracking", and to be able to modify the speed of either transmission, relative to the other, to effect steering of the vehicle. Control systems for dual path hydrostatic drive systems were originally hydraulic, but more recently, as the controls have become more complex, an increasing portion of the control logic has been accomplished electrically. An example of such a control system is illustrated in U.S. Pat. No. 3,914,938, assigned to the assignee of the present invention.
In the prior art electrical steering control systems for dual path vehicles, it has been typical to use the speed command signal for excitation of the steering signal generator, effectively multiplying the steering command and the speed command to provide an output signal having a gain dependent upon the speed command. As a result, there is no signal available, as an input to the system logic, which actually indicates the desired or commanded "steering ratio", i.e., the relationship of the speeds of the two tracks.
Accordingly, it is an object of the present invention to provide an electrical steering control system for a dual path vehicle in which the steering input results in the generation of a steering signal representative of the commanded steering ratio, uneffected by instantaneous vehicle speed, such that the commanded steering ratio is available as an input to the subsequent system logic.
Typically, dual track vehicles are operated in reverse a substantial portion of the total operating time. Therefore, it is important that such vehicles be easy to steer during both forward and reverse operation. Conventionally, the steering of dual track vehicles has been accomplished either by a pair of steering inputs (as in U.S. Pat. No. 3,914,938), such as a pair of foot pedals, or by a single steering input (as in U.S. Pat. No. 3,848,690), such as the well known joy-stick. In either case, it has been generally acceptable, for example, for right-hand steering actuation (depressing the right foot pedal or moving the joy-stick to the right) to result in slowing the hydrostatic transmission driving the right track, whether the vehicle is moving in the forward direction or in the reverse direction.
On many dual path vehicles, T-handle control is preferred because it is simpler to operate than the systems which have a single control for speed and a pair of foot pedals for steering. T-handle control is also preferred over a joy-stick in many applications where the rough terrain over which the vehicle travels makes it difficult to control the joy-stick. However, the prior art electrical steering control systems for dual path vehicles are unsatisfactory for use with T-handle controls, because rotation of the T-handle in one direction will always reduce the speed of the transmission driving the right track, while rotation of the T-handle in the other direction will always reduce the speed of the transmission driving the left track.
Accordingly, it is an object of the present invention to provide an electrical steering control system for dual path vehicles capable of being used with T-handle controls, such that the vehicle may travel generally perpendicular to the T-handle orientation for both forward and reverse directions of vehicle operation.